Recently, as a battery that meets the expectations for miniaturization, lightness, and high capacity, a non-aqueous electrolytic solution-based secondary battery such as a lithium ion battery has been suggested and put into practical use. The lithium ion battery includes a positive electrode and a negative electrode which have properties capable of reversibly intercalating and deintercalating lithium ions, and a non-aqueous electrolyte.
The lithium ion battery is small in size, is light in weight, and has high energy compared to secondary batteries such as a lead battery, a nickel-cadmium battery, and a nickel-hydrogen battery in the related art, and thus the lithium ion battery has been used as a power supply of a portable electronic apparatus such as a cellular phone, and a note-book type personal computer. In addition, recently, an examination also has been made for a high-output power supply of an electric vehicle, a hybrid vehicle, and an electric tool. High-speed charge and discharge characteristics have been demanded for the electrode active material of the battery that is used as the high-output power supply.
Therefore, from the viewpoints of high functionality, high capacity, low cost, and the like of the above-described secondary battery, as a positive electrode active material, various kinds of materials have been examined. Among these, an olivine-type phosphate-based electrode active material represented by LiMnPO4 or LiCoPO4 has attracted attention as an electrode active material from the viewpoints of safety, abundant resources, and low cost.
However, with regard to the olivine-type phosphate-based electrode active material, a problem of poor material utilization under low-rate discharge conditions has been frequently pointed out (refer to Non-Patent Document 1 and the like).
As one problem of the poor material utilization, a problem of slowness in Li diffusion inside an active material, which is derived from a structure of the olivine-type phosphate-based electrode active material, is an exemplary example.
In the olivine-type phosphate-based electrode active material, it is known that the Li diffusion inside the active material occurs only in a b-axis direction of a crystal lattice while being accompanied with phase conversion of two phases of LiMPO4 (M represents one or more kinds selected from a group consisting of Mn, Co, and Ni) and MPO4 (M represents one or more kinds selected from a group consisting of Mn, Co, and Ni) (refer to Non-Patent Document 2), and it is described that the olivine-type phosphate-based electrode active material is unsuitable for high-speed charge and discharge.
As an effective method to solve the problem, a method of shortening the crystal lattice length of LiMPO4 (M represents one or more kinds selected from a group consisting of Mn, Co, and Ni) particles in a b-axis direction for the purpose of shortening a Li diffusion distance in the particles, a method of making primary particles of the particles have a sheet-shaped crystal shape that is thin in the b-axis direction, or a method of making LiMPO4 particles fine for the purpose of increasing a reaction area between Li and LiMPO4 particles are exemplary examples.
As a method of making the LiMPO4 particles fine, a method of making the LiMPO4 particles fine by mechanical pulverization is general (refer to Patent Document 1 and the like).
In addition, as another method, a method of making particles fine using polyhydric alcohols such as glycols and polyols which have a high boiling point is suggested (refer to Patent Document 2 and the like).
This method is a method of allowing LiMPO4 particles to precipitate while heating a sufficient amount of precursor in the polyhydric alcohols such as glycols and polyols which have a high boiling point.